A liquid crystal display (LCD) comprises liquid crystal material contained between two conductive substrates which are sealed at their periphery. The liquid crystal substrates are conventionally comprised of plates of glass or plastic which are selectively coated with an electroconductive film which in turn is coated with an alignment layer.
It is the function of the alignment layers to orient the liquid crystal molecules in the LCD in a homogeneous manner. The capacity of the alignment layer to orient liquid crystal material is created conventionally by unidirectionally rubbing the layer with a soft material such as cotton cloth or paper. Liquid crystal molecules adjacent to the surface will then align parallel to the rubbed lines on the surface. When the rubbed lines on alignment layers in an LCD are in parallel orientation, the liquid crystals are in parallel homogeneous alignment. Twisted liquid crystal devices are formed by positioning the alignment layers in the device so that the rubbed lines are at right angles.
A number of materials have been proposed for the formation of alignment layers. These include a layer of silicon dioxide deposited at an incidence angle of about 5.degree.; polyimides; polyamides; organosilanes; and poly(vinyl alcohol). Each of these materials has certain disadvantages. Slant-evaporated silicon dioxide, for example, is comparatively difficult to apply. Polyimides, although stable to processing temperature of up to about 300.degree. C., are electrically insulating. It is difficult to make electrical contact through a layer of polyimide resin without removing a portion thereof, which may be troublesome once it is cured. Also, certain epoxy resin sealants for the periphery of the LCD do not adhere well to polyimide resins.
Polyamides, like polyimides, have a comparatively high cure temperature and also are electrically insulating. Further, polyamides undergo hydrolysis more readily than polyimides.
Organosilanes are very good alignment materials which can tolerate temperatures of from 200.degree. up to about 450.degree. C. The organosilanes, however, have essentially no blocking capacity against the passage of moisture and ions, particularly sodium ions, which can migrate from glass substrates into the liquid crystal material. This is a decided disadvantage in view of the recognized instability of liquid crystal materials to these impurities.
Poly(vinyl alcohol) is also a very good alignment material. However, it is stable only to from about 85.degree. to 95.degree. C. Hence, it cannot be used with any sealant composition which must be cured above these temperatures. Also, poly(vinyl alcohol) is only an adequate barrier against the passage of moisture and ions.
In accordance with this invention, a material has been found which does not suffer the disadvantages of previously used alignment materials.